Following are the main source files of this application

/*
 * @author Rajat Gupta
 * @version 1.0
 *
 * Name of the Application        : GetMessage.java

 * Development Environment       : Oracle9i JDeveloper
 * Creation/Modification History  :
 *
 *
 *   Rajat Gupta        28-May-2003        Created
 */

package oracle.otnsamples.oc4jjms;

// IO Imports

import java.io.IOException;
import java.io.PrintWriter;

// Util Imports
import java.util.HashMap;

// Naming Imports
import javax.naming.InitialContext;

// Servlet Imports
import javax.servlet.ServletConfig;

import javax.servlet.ServletException;

// HTTP Servlet Imports
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


/**
 * This Servlet reads the message from the queue. If any message is available
 * for the client, then it sends the message or else returns null.
 *

 */
public class GetMessage extends HttpServlet  {

  /**
   * Content type of the response
   */
  private static final String CONTENT_TYPE = "text/xml; charset=ISO-8859-1";

  /**
   * This HashMap stores the objects of the users currently logged into the
   * application. A registered user gets the message through this object.
   */
  private HashMap currentSubscribers = new HashMap(5);


  /**
   * Init method of the Servlet
   *
   * @param config
   *
   * @exception ServletException
   */
  public void init(ServletConfig config) throws ServletException {
    try{
      super.init(config);
    }catch(Exception e){
      // Print the exception on the console
      e.printStackTrace();

    }
  }

  /**
   * This method is invoked when a client sends a message to this servlet. This
   * method performs various operations of which the important ones are
   * described as follows :
   *    1) The user can login to the Application.
   *    2) The user receives the message through this method.
   *
   * While registering a user, this method creates a subscriber object on
   * behalf of the user. The subscriber object is then stored in the HashMap
   * from where it is retrieved for this users subsequent requests.
   *
   * @param request ServletRequest Object

   * @param response ServletResponse Object
   *
   * @exception ServletException
   * @exception IOException
   */
  public void doGet(HttpServletRequest request, HttpServletResponse response)
                                          throws ServletException, IOException {
    // This variable stores the message that is to be returned to the
    // client.
    String message;

    // Set the Content type
    response.setContentType(CONTENT_TYPE);

    try{
      if (request.getParameter("doRegister") != null){

        // This loop will be accessed for the first request of the user. This
        // loop registers the user into the application.

        //This variable stores the ID of the currently connected client.
        String clientID = request.getParameter("id");

        if (currentSubscribers.containsKey(clientID)){
          // This loop checks if the user is already registered

          message = "AlreadyCreated";
        }else{
          // Its a new user. This loop will create the subscriber object.

          // Create the subscriber object for this client
          ChatSubscriber subscriber = new ChatSubscriber();
          subscriber.createSubscriber(new InitialContext(), clientID);


          // Store the client object in a HashMap for future reference.
          currentSubscribers.put(clientID, subscriber);

          message = "Created";
        }
      }else if (request.getParameter("checkIfCreated") != null){
        // This loop will be accessed if the client selects that he/she is
        // already registered with the application. This loop will check
        // if he/she is really registered.

        // Get the Client ID.
        String clientID = request.getParameter("id");

        // Get the Subscriber object
        ChatSubscriber subscriber =
                  (ChatSubscriber)currentSubscribers.get(clientID);

        if (subscriber == null){

          // The user is not already registered
          message = "NotCreated";
        }else{
          // The user is already registered
          message = "Created";
        }
      }else if (request.getParameter("id") != null){
        // This loop connects to the topic and checks if any message is
        // present for the user.

        // Get the client ID
        String clientID = request.getParameter("id");

        // get the Subscriber object
        ChatSubscriber subscriber =
                  (ChatSubscriber)currentSubscribers.get(clientID);

        // Get the message
        message = subscriber.getMessage();

      }else{
        // This loop will be accessed if some problem occurs and the server
        // could not access the Client ID.

        message = "Unexpected Problem. Please Login again.";
      }

      // Initialize the PrintWriter object
      PrintWriter out = response.getWriter();

      // Send the message to the User.
      out.print(message);

    }catch(Exception e){
      // Print the exception on the console
      e.printStackTrace();
    }
  }

  /** This method redirects the client request to the doGet() method.

   *
   * @param request ServletRequest Object
   * @param response ServletResponse Object
   *
   * @exception ServletException
   * @exception IOException
   */
  public void doPost(HttpServletRequest request, HttpServletResponse response)
                                         throws ServletException, IOException {
    doGet(request, response);
  }

}